Aircraft doors must meet several requirements. On the one hand, during operation of the aircraft, the doors must be tightly locked and they must be capable of taking up all loads to which the doors may be exposed. On the other hand, it is necessary that such doors can be opened rapidly and simply in case of an emergency. For taking up all loads aircraft doors assume a formlocking position relative to the aircraft body structure in the closed state of a door in which the door is additionally locked. Doors for aircraft having a pressurized cabin must also be capable to withstand the internal cabin pressure during flight. In connection with passenger aircraft, the requirement must be met that each passenger door is equipped with an emergency slide which must be activated and deployed in an emergency through the operating elements of the door itself.
In order to establish a formlocking connection between the door and the aircraft body structure, and for transmitting the forces resulting from the internal cabin pressure, there are two door closure principles conventionally employed. One principle is the so-called contact principle in which the door is brought into sealing contact with the door frame in the aircraft body structure. The other principle is the so-called toggle principle in which the door is closed in a sealing manner by means of toggle levers.
Doors constructed to operate in accordance with the contact principle comprise along their lateral rims or edges contact hardware. For closing such doors the doors are lowered subsequent to their being tilted into the body structure. The lowering of the closed door by a determined distance is such that the door hardware components contact and rest against respective body contact components and the cooperation of these components makes sure that all outwardly directed forces that are effective on the door are taken up safely.
Doors constructed to operate under the toggle lever principle are equipped along their lateral edges by rotatable toggles which, during the closing movement of the door, extend alongside the door edges and which enter into respective recesses in the door frame. In order to close the door the toggles are rotated within the recesses so that they extend crosswise rather than lengthwise to the door edge, whereby the required formlocking is achieved with the aircraft body structure.
In both types of doors the operational steps, such as closing, locking, activating of the emergency slide, and the respective opposite operations are preformed manually. U.S. Pat. No. 4,720,065 (Hamatani) discloses a door operating system in which the door closes in accordance with the above mentioned contact principle. The operating mechanism arranged inside the door according to U.S. Pat. No. 4,720,065, is so constructed that an operating motion introduced by means of a hand lever arranged on the side of the door facing into the cabin, is transmitted to a lifting shaft, to a closing flap, and to a locking mechanism of the emergency slide. The lifting shaft makes sure that upon opening the door, the door is lifted and that upon closing the door, the door is lowered. The operation of the known door will now be described by way of example, with reference to the sequence of operations occurring during the lifting and during the lowering of the door. The lifting shaft comprises a crank connected laterally to each end of the lifting shaft. These cranks enter with their crank arm into respective guide tracks when the door is swung into the closing position. For lowering the door the lifting shaft is then rotated with the hand operating lever of the door in such a way that the two crank arms travel upwardly on a respective circular path relative to the door, whereby the crank arms bear against the guide tracks so that the door is lowered downwardly, overcoming the friction of the door seal. For opening the door, the latter must first be lifted in the opposite direction by rotating the lifting shaft. It is desirable to reduce the force necessary, especially for lifting the door, to minimize the physical strength of a person required to operate the door. For this purpose a weight compensation device is provided for cooperation with the lifting operation. The compensating device comprises a spring which is cocked while lowering the door and which helps lifting the door prior to opening the door. The mechanism required for performing the just described several functions is very complex and comprises numerous bearings, shafts, levers, journals, and pivots as well as a cam drive. The situation becomes aggrevated when at least some of the components must be provided in duplicate to meet so-called failsafe requirements. Due to the large number of mechanically effective components and elements, the production costs for such doors are quite high. Further, it has been noted especially in connection with larger passenger doors that the operation of such doors does require a relatively large bodily strength for lifting these doors even if the compensating device is provided.
The above described facts relating to an aircraft door constructed according to the contact principle, apply substantially also to doors constructed for operation in accordance with the toggle principle. Even though the lifting and lowering is not involved in doors operated in accordance with the toggle principle, a substantial technical effort and expense is still necessary in view of the fact that even toggle locked doors require a substantial number of mechanical structural components. Another disadvantage of the toggle principle is seen in that the toggle levers enter into the respective recesses only when the door is precisely positioned. Without such precise positioning the toggle levers cannot be operated. The required precision construction also increases the effort and expense for producing such doors so that they function without fault.